But other colours will have luminance less than 1.0. For example, a bright blue (0.2,0.4,1.0) has a luminance of only 0.21x0.2 + 0.72x0.4 + 0.07x1.0 = 0.4

If you choose to apply a curve that tries to raise a luminance of 0.4 to a higher value, it can't be done to our bright blue colour because the blue component is already 1.0, which is the maximum that can be represented. So, the blue channel will be clipped, which changes the colour. In fact, this is likely to happen to any colours that are close to the maximum brightness than can be represented, if the tone curve is anywhere raised above its initial value.

Sorry about the maths, but I can't think of any other way to explain it.